This invention relates to the testing of semiconductor wafers and, more particularly, to photoluminescence measurements of double heterostructure (DH) wafers.
In the prior art, the position of a p-n junction in semiconductor material is typically determined either by EBIC (electron beam induced current) techniques or by chemical staining techniques, both of which are destructive methods.
Although photoluminescence (PL) is a powerful tool for nondestructive evaluation of semiconductor material, it has not been used for determining p-n junction misplacement. Rather, it is often used for analytical characterizations such as determining bandgap energy and approximate carrier concentration. Scanning PL is a common technique for revealing defects and nonradiative centers in epitaxial layers. In studying nonradiative centers in double heterostructure (DH) material, W. D. Johnston, Jr., Appl. Phys. Lett., Vol. 24, page 494 (1974), found that defects can induce "large dark spots" (LDS) of diminished PL for distances of the order of 100 .mu.m around the defects. C. H. Henry et al, J. Appl. Phys., Vol. 48, page 3962 (1977), later showed that LDS occur around defects within a DH with a p-n junction. Photoexcited minority carriers diffuse across the junction and drift to the defect where they recombine nonradiatively. Others have studied the excitation dependence of the active layer PL in AlGaAs laser heterostructures. They found a highly nonlinear variation of the PL signal with the excitation intensity at low power density levels, which they have attributed to nonradiative surface recombination at the p-n junction.